Control apparatus



May 15, 1945.

' w. H. GILLE ETAL CONTROL APPARATUS Filed Aug. 28, 1941 I5 Sheets-Sheet l 56 5 R. 53 trua. TEMPERATURE RESPONSIVE ELEMENT Egg Fig.1

INVENTORS Willis 14.6mm f.

Jchmv sigfordJ ATTORNEY May 15, 1945.

Filed Aug. 28, 1941 3 Sheets-Sheet 2 1 F1 .2 m 206 2. 2'5 207 g INVENTORS Willis H. Gi11 z & John. VI Sigford.

ATTORNEY May 15, 1945.

W. H. GILLE ETAL CONTROL APPARATUS Filed Aug. 28, 1941 3 Sheets-Sheet 3 [EREP' IBI Q AMPLIFIER- A [H3 h x INVENTOR Willis H. Gina. &

John. \L sigforb av a M ATTORNEY Patented May 15, 185 7 Um'rsn STATES" PATENT OFFICE oom'aor. Arraaa'rus g mm. a. ems. St. Paul. sums, and John v. su lordI Wabash, d., assignors to Minneapolis- Honeywell tor Company, Minneapolis, Minn" a corporatlon of Delaware I Application August 28,194L8erlallim408594 8Clalms. (cuss-o1) The present invention to condition control systems, and particularly to control systems including a condition responsive element having acharacteristic variation inimpedance in ac-' cordance with said condition.-

It been desired sen-.

sitivity of condition systems. suc temperature control The principles cioperation oi extremely sensitive sponsive elements resistors or material an jappreciame coeificient of resistance. etc; also'well known. The

.use' of -"such sensitiveelelnentsiin commercial impractical because the changesjin'suchelements in re- .sponsetochangesintemperaturearequitesmall.

Hence the [means required to amplify "such changes in order to produce sufiicient amounts of power to control furnaces, cooling systems, Y

etc, have been quite complicated and expensive.

Aturther obiectistoconstructanimprowed sensitive element for outdoor use of a temperature sensitive element darkened to make it sensitive to radiant heat, such as sunlight. A iurther obiect is to construct an improved sensitive element for outdoor use which is heated so as to make it sensitive to wind velocity. A turther object is to provide improved means for heating an element connected in a bridge circuit by means of a current flowing through the element itself, wherein the heating current is prevented from disturbing the balance oi the bridge circuit. Another object of the present invention is to provide an improved electronic amplifier which is adapted to receive the output of a bridge cir- Itis therefore an object of the present invention i to construct an improved condition control system including a sensitive condition responsive element, means for amplifying changes of said element, and control means operated thereby,

which shall be'more emcient andless expensive than those oithe priorart,"

It is a further object o! the present invention to provide, in such a control system, an improved bridge circuit for connecting a plurality of condition responsive elements so as to secure a con- 'cuit in its input circuit and to pi'oduce a potential suitable for controlling the operation of a l motor in its output circuit.

Another object of the present invention is to provide an electronic amplifier ior supplying one phase 0! a split-phase motor, wherein the phase of the amplifier output current is varied in accordance with the phase 0! the input current.

and wherein means is provided to limit the displacement of said output current phase from. a predetermined normal. 7

A further object of the invention is to construct an amplifier oi the wpe described. the

phase displacement limiting means consists oi trol system which is compensated for the variation of a plurality 0! conditions.

A further object of the invention is to provide, in a bridge circuit including two remotely located condition responsive elements in a single arm thereof. improved means for compensating said bridge circuit for changes in the resistance of the leads connected to said remote elements.

A further object is to provide a condition responsive element for compensating such a control system,-and to provide means for securing a non-linear compensating efiect from said element. A still further object is to construct a condition responsive non-linear compensating device for a resistance bridge. comprising an element having an appreciable temperature coefficient of resistance and a rheostat operated by temperature responsive means connected in parallel with said element.

Another object oi'the invention is to construct tive to temperature, radiant-1heat,"and wind velocity. v

an automatic volume control circuit. A further object of the invention is to construct an amplifier oi the type described wherein a limitation of the phase displacement of the output current is obtained by implying the final output stage i an improved, sensitive element adapted to be mountedoutdoorsandarrangedso astobesensiwith unfiltered rectified alternating current.

Another object of the present invention is tc provide an improved temperature control system for a building including a first element responsive to temperature, a second element responsive to a condition of a temperature changing means and a -sthird element responsive to outdooi weather conditions.

Another object oi the invention is to construci a condition control system including a pluralit: of condition responsive bridge circuits, contro means associated'with eachcircuittobe con trolled by the output thereof, and a single ampli' fier means adapted selectively to connect any a said bridge circuits with its associated contro means. A further object is to construct a tim1 operated switching device for successively con nectingeachoisaidbrldge circultswithits asso Another object or the invention is to provid an air conditioning system for controlling the air supply to a plurality of zones, including sensitive condition responsive means associated with each zone, air conditioning means associated with each zone, a single amplifier, and switch means for selectively and successively connecting each said condition responsive means to its associated air conditioning means through said amplifier.

Other objects and advantages of my invention will appear from a consideration of the accompanying claims, specifications and drawings in which Figure 1 represents a temperature control system embodying certain features of my invention,

Figure 2 represents an electronic amplifier circuit which maybe used in the system of Figure 1,

Figure 8 represents a modified form of electronic amplifier circuit which is also applicable to the system of Figure 1, and

Figure 4 shows an air conditioning system embodying certain features of my invention.

Figure 1 Figure 1 shows a space I0 heated by a radiator I. Heating fluid, which may be, for example, steam or hot water, is supplied to the radiator through a pipe l2 and exhausted fluid is drawn of! from the radiator through a return line l3. The fluid is heated by a furnace |4 provided with a burner l5, to which fluid fuel is supplied through a pipe I5.

This supply of fuel through the pipe I5 is controlled by a valve whose position is modulated between open and closed positions by an operating mechanism 2|. The mechanism 2| includes a reversible motor 22 of the split-phase type having a pair of field windings 23 and 24. Winding 24 is energized constantly through suitable connections with a secondary winding 25 of a transformer 25. Energization of winding 23 is controlled by an amplifier schematically indicated at 21. Suitable phase shifting means may be connected in series with one of the windings 23 or 24. In the system shown a condenser 28 is connected in series with winding 24.

Transformer 25 is provided with a primary winding 30 connected to power supply lines 3| and 32. In addition to the secondary winding 25, the transformer 25 also has secondary windings 33 and 34. Also connected to the supply lines 3| and 32 is a transformer primary winding 35, Primary winding 35 is a part of a transformer 35 having a secondary winding 31 which supplies a bridge circuit generally indicated at 40.

Bridge circuit has input terminals 4| and 42 and output terminals 43 and 44. Output terminal 44 is the point of contact of a slider 45 with a slide wire resistance 45. Slider 45 is moved along the slide wire 45 by the motor 22 as it operates the valve 20.

Transformer secondary winding 31 is connected to bridge input terminals 4| and 42 by conductors and 5|, respectively, thereby forming with said conductors 50 and 5|, the input circuit of the. bridge 40.

The upper left-hand arm of the bridge circuit 45 connects input terminal 4| with output terminal 45, the latter being located remotely from the bridge circuit 45. Thisupper left arm includes a fixed resistance 52, a variable resistance 58, and a long conductor 54 connecting variable resistance 55 with the remotely located terminal 43. The function of variable resistance 53 is to adjust the setting of the bridge. That is,'by varying the amount of resistance in the upper left arm or the bridge. the resistance necessary in the other arms of the bridge in order to produce a balanced condition is also changed.

The upper right arm of the bridge 40 connects output terminal 43 with input terminal 42 and includes three temperature responsive resistance elements 55, 55 and 51. This arm of the bridge circuit may be traced from terminal 42 through resistance element 55, a conductor 55, resistance element 55, a conductor 50. resistance element 51, and a conductor 5| to input terminal 42.

The temperature responsive resistance element 55 is located'outside the building containing the space iii. The function of resistance element 55 is to cause an unbalance of the bridge circuit 40 in accordance withoutside weather conditions affecting the operation of the temperature control system. The resistance element 55 may be formed of nickel or some other substance having an appreciable temperature coefficient of resistance, so that its resistance varies directly as the outside temperature.

We have found that by enclosing the resistance element 55 in some substance having a comparatively dark surface, its resistance may be made responsive to radiant energy conditions. For example, in bright sunlight, the dark surface will absorb radiant energy from the sun and cause an increase in the temperature of any elements enclosed within the dark surface. Since the presence of bright sunlight outside decreases the amount of heat which the furnace l4 must supply to the space l5, it is desirable that the operation of the control system be corrected in accordance with the outside radiant heat condition. We have found that by wrapping our outside temperature responsive element in kraft paper of a suitable shade, suitable radiant heat absorptive characteristics are imparted to the element.

It is also desirable that the resistance element 55 be made responsive to wind velocity, as the amount of heat required from the furnace l3 will be greater when a high wind is present outside the building than when the air is comparatively still. We have made the resistance element 65 responsive to wind velocity by heating it artificially. When a high wind is blowing, a large proportion of this artificial heat is carried away from the resistance element 55, thereby decreasing its temperature below normal. The element 55, therefore, reacts to the presence of a high wind in the'same manner that it does to a decrease in temperature.

Instead of heating the resistance element 55 externally to make it respond to wind velocity, we find it more convenient to supply heat by means of an auxiliary bridge circuit 52, having input terminals 53 and 54 connected to the terminals of transformer secondary winding 34 by conductors l0, and II, respectively. Input terminal 53 is the mid-point of resistance element 55. The two ends of element 55 form two of the arms of the auxiliary bridge circuit 52. The other two arms of bridge circuit 52 are formed by a pair of coils 55 and 51 oppositely wound on a common core- 55. One terminal of each of the coils 55 and 51 is connected to the input terminal 54 of auxiliary bridge circuit 62. The opposite terminals of coils 55 and 51 are connected to terminals 43 and 55 of temperature responsive element 55. Current flowing in the auxiliary bridge circuit 52 passes through windings 55 and 51 in opposite directions. Since these coils are oppositely wound, the-magnetic fluxes produced asracss in the core 88 by the current nowing in the coils 88 and 81 are in the same direction. The coils 58 and 81 therefore oiler relatively little imp dance to the how of current in the bridge circuit 8i.

The coils 88 and 81 are. however, connected in series with respect to any current flowing in the bridge circuit 40. since the coils 85 and 81 are wound in opposite directions. any current passing through the two coils in series produces in one coil a magnetic flux which reacts on theother coil to oppose the flow or this same current. It is therefore apparent that the coils 88 and 61 present a substantially infinite impedance to the how of current from the bridge circuit 45.

It should also be apparent that current from the bridge 82 cannot flow in the bridge 45 as the bridge 62 is always balanced, and as far as the bridge 82 is concerned terminals 48 and 85 are at the same potential. The bridge 82 is always balanced because the coils 85 and 81 are of constant impedance and because any variation in the resistance of element 55 is automatically balanced since half of this resistance is in each of the two adjacent arms of the bridge.

'I'o summarizethe operation 01 the temperature responsive element 55, it may be stated that any outside weather condition which. tends to cause an increase in the heat loss from the space HI, also tends to cause a decrease in the resistance of element 55. Such conditions may be a drop in outside temperature, a decrease in the outside radiant heat, or an increase in wind velocity.

The function of temperature responsive element 56 is to introduce an unbalancing eflect into the bridge circuit 40 in accordance with changes in temperature within the space Ill. Such changes in temperature within the space indicates the necessity of increasing or decreasing the amount of heat supplied to the space by the furnace I4.

The function of the temperature responsive resistance element 51 is to introduce an unbalance into the bridge circuit 40 in accordance with connecting the remote element with the bridge. This is conventionally done by so connecting the remote element that the two conductors connecting it with the bridge are in diiierent adjacent arms of the bridge. In this way, each of the two pairs of opposite arms contain one of the lon conductors. Since these conductors lie physically close to each other practically throughout their length, any change in the ambient temperature oi the medium through which they pass affects both arms of the bridge equally and thereiore produces no unbalancing eiiect. Such a connection is conventionally accomplished by locating one oi the terminals of the bridge circuit at a point near the remote element. as for example, the terminal 48 in Figure 1. Because of the location or output terminal 43 at this point the conductor 54 is in the upper left arm of the bridge circuit while the conductor 58 is in the upper right arm oi. the bridge circuit.

It has not previously been realized, however, that complete compensation for the lead resistance could be secured when two remotely located resistance elements were connected in the same arm of a bridge circuit. we have shown such a circuit in Figure 1. in which both resistance elements 55 and 51 are located remotely from the main part of the bridge circuit 48. The manner in which leads 54 and 58 compensate the bridge circuit for each other's resistance has been explained above. with regard to resistance element 51. the lead I1 is in the lower right arm of the bridge circuit while the conductor 50 is in the upper right arm or the bridge circuit. Therefore. the resistance or each of these conductors opposes the eflect oi the other on the unbalance of the bridge circuit.

' ductor which extends back to the location of the the temperature of the heating fluid at the output of the furnace N. This fluid in the furnace l4 will be delivered to the space i0, and a change in its temperature is reflected in the change in resistance of element 51 so that the delivery of the heated fluid t0 the Space I0 is anticipated b the system.

The lower left arm of bridge circuit 48 connects input terminal 4| with output terminal 44 and includes a conductor 12, a fixed resistance 13, a conductor 14, and that part of slide wire resistance 46 between its left-hand terminal and the point of contact 44 of the slider 45.

The lower right-hand arm of bridge circuit 40 connects output terminal 44 with input terminal 42 and includes that part of slide wire resistance 46 between slider '45 and the right-hand terminal of resistance 48, a conductor 15, a fixed resistance 18, and a conductor 11.

A fundamental characteristic of bridge circuit is that, during balanced conditions, the product of the resistances of any two opposite arms or the bridge is equal to the product of the resistances of the other two opposite arms of the bridge. It is customary, when using a bridge circult to measure the resistance of an element located remotely from the other part of the bridge, to compensate the bridgecircuit so that the balance will not be disturbed by the long conductors main part of the bridge circuit. By opposite terminal of the remotely located element is meant a terminal such as in Figure 1, which is opposite to terminal 48, as far as the resistance element 55 is concerned. In other words, the necessity of using the conductors 58 and 58 of Figure 1 was not previously realized. As far as the applicant's system is concerned, temperature responsive resistance element 58 could be located in the lower left arm of the bridge circuit 48 as well as in the upper right. Regardless of the particular arm in which the resistance element 58 is connected, the conductors 58 and 80, extending physically parallel to the conductors 54 and I1, respectively, between the respective remote resistance elements 55 and 51 and the main part of the bridge circuit, are necessary in order to compensate the bridge circuit for the resistance of the lead.

In bridge circuits of the prior art, whenever two remote resistances were used in a single bridge arm, it was customary to connect the two remote elements directly together. A comparable result would be obtained by connecting our terminal 85 with the right-hand terminal oi! resistance 51 directly. It should be apparent that such a connection destroys any possibility of compensating the bridge circuit for the lead resistances by the method outlined above.

The above described means for compensating the bridge circuit for the lead resistances is claimed in our copending divisional application Serial No. 560,662, filed October 27, 1944.

Operation Figure 1 When the parts are in the position shown in the drawing, the bridge circuit 4| is balanced. and the furnace i4 is supplying an amount of heat to the space It which is just sufllcient to balance the heat losses, thereby maintaining the space at the temperature which the system has been set to maintain by adjustment of the variable resistance 52.

Let it now be assumed that there is a decrease in resistance oi one of the three temperature sensitive elements 55, 48 or 51. With regard to outdoor element Iii. such a decrease in resistance indicates the existence of a condition which will cause an increased heat loss from the space It. In the case of resistance element N, such a decrease indicates the' presence of such an increased heat loss. 0n the other hand, such a decrease in the resistance oi element 51 indicates that the amount of heat supplied to the space i0 is about to be decreased. In any event, a decrease in resistance of any one of these three elements indicates that the amount of fuel supplied to the burner i5 should be increased if the temperature of the space I0 is to be maintained at its predetermined value.

For convenience in describing the operation of the system, the instantaneous polarity of the source of electrical energy will be assumed to be that indicated by the legend in the drawings. In that case, any decrease in resistance of the upper right-hand arm of bridge circuit 40 causes the potential of output terminal 42 to become more negative than the potential of output terminal 44. This change in potential of output terminal 43 causes a current to flow in the bridge output circuit in a direction from terminal 44 through slider 45, a conductor 80, amplifier input terminal 84, the input circuit of amplifier 21, amplifier input terminal 85, and a conductor 8| to output terminal 43.

Power is supplied to amplifier 21 from transformer secondary winding 33 through conductors 82 and 83. The current flowing in the input circuit of amplifier 21 produces a greatly amplified current in its output circuit. The amplifier output current energizes winding 23 of motor 22, fiowing through a circuit which may be traced from output terminal 86 of amplifier 21 through conductors 90 and Si, winding 23, and a conductor 92 to output terminal 81 of amplifier 2'l.

Winding 24 of motor 22 is constantly energized through a circuit which may be traced from the upper terminal of transformer secondary winding 25 through a conductor 93, conductor 9|, winding 24, a conductor 94, condenser 28, and a conductor 95 to the lower terminal of transformer secondary winding25. Conductor 9i, which is connected to the common terminal of windings 23 and 24, is grounded, as at 95.

Since the winding 24 is directly connected to transformer secondary winding 25, the time phase of current flowing through the winding 24 is fixed with respect to the time phase of the potential supplied by the winding 25. The condenser 28 is chosen so that the current through winding 24 leads the supply potential by approximately 90 electrical degrees. The time phase of the current flowing in winding 23 of motor 22 depends on the time phase of the output current of amplifier 21. The time phase of this output current depends in turn on the time phase oi the amplifier input current, which is determined by the instantaneous direction of now 0! current between output terminals 48 and 44 oi bridge circuit 4|. The output current oi the bridge 40 may be either in phase or 180 electrical degrees out oi phase with the potential supplied irom lines H and 82. The particular phase relationship depends, as previously stated, upon the instantaneous direction oi flow of current between terminals 42 and 44,

During the conditions at present under consideration, it has been indicated that a current is flowing in the output circuit oi bridge 40 in a direction from terminal 44 to terminal 43. Le it be assumed that the connections are such that when current is flowing in this direction, it is 180 degrees out of phase with the supply line voltage. The output current oi amplifier 21 is likewise approximately 180 degrees out of phase with the supply line voltage. The current in winding 23 therefore diflers in phase from the supply current by 180 degrees, while the current in winding 24 leads the supply potential by only degrees. This difference in phase of the current in windings 22 and 24 causes split-phase motor 22 t be driven in such a direction as to open the valve 20 wider. It may thereiore be seen that an unbalance of bridge circuit 40 in such a direction as to indicate the need for an additional supply of heat to space I0 causes operation of motor 22 in such a direction as to increase the supply of fuel to the burner l5.

As motor 22 drives the valve 20 in opening direction, it also moves slider 45 to the right along slide wire 46. This motion of slider 45 causes the potential of output terminal 44 to become more negative. The opening motion of valve 20 and the movement oi slider 45 to the right continues until the motion of slider 45 has caused a change in potential of output terminal 44 sufficient to balance the change in potential of terminal 43. At that time current ceases to fiow in the input circuit of amplifier 21. Current also ceases to flow in the amplifier output circuit and the winding 23 is thereiore deenergized, stopping the motor 22.

Now let it be assumed that an increase in resistance takes place in any one or the three temperature responsive elements 55, 56 or 51. This increase in resistance indicates a necessity of decreasing the supply of heat to the space 10, and causes output terminal 43 to become positive with respect to terminal 44. The current therefore flows through the output circuit of bridge 40 in a direction from terminal 43 to terminal 44. This current, flowing through the input circuit of amplifier 21, causes an amplified current to fiow in the winding 23 of motor 22. The phase of this current in winding 23 will be opposite that under the conditions previously discussed, because the instantaneous direction of flow through the amplifier input circuit is opposite to that under the previous conditions. The current in winding 23 is therefore in phase with the supply potential. As the current in winding 24 is leading the supply potential in phase by 90 electrical degrees, the motor 22 will operate in a direction opposite to that which it rotated under the previous conditions.

The rotation of motor 22 will therefore cause movement of valve 20 towards closed position and movement of slider 45 to the left along slide wire 46. The movement of valve 20 in a closing direction will decrease the supply of heat to the space l0, and this movement will continue until sneer 4| reaches-a position such umterminal 44 is at the same Potential asterminal 43. When this pofltion is reached, current ceases to flow in the output circuit of bridge, winding 28 of motor 22 is therefore'deenergized'and the motor comestorest.

Itshouidbeapparenttothoseskilledinthe art that the condenser 28, or other phase-shiftin'gmeans, maybe connected in series with;either winding of motor 22, without afiecting the-operationotthesysteni.

Figure 2 illustrates an electronic circuit generally referred to as 221,-which corre sponds generallyto the amplifier 21 of Figure 1, and-may be used therefor. The terminals'284 and 285 are the input terminals of the amplifier 221 and correspond to input terminals 84 and 85 of amplifier 28 in Figure 1. Likewise, output terminals 288 and 281 correspond to output terminals 88 and 81 in Figure 1, and power supply terminals 288; and 288 correspond to power supply terminals "and 88 of Figure 1.

Amplifier 221 consists generally oftwo stages of amplification using triodes 28.8 and at con-' nected in said stages with a final push-pull stage of amplification using a twin triode 282. The

amplifier 221 also includes a'power supp and rectifier circuit of conventional type using a rectifiertube 203. v I

The power supply circuit includesa transformer 204 having aiprimaryjwinding '205and a pair of secondary 285 and 281. Primary winding 205 is connected to power supply terminals 288 and 288.. The rectifier tube 283'has a pair of anodes 2l8 .and2li cooperating with a common cathode,2l 2.1 The anodes 2 and 2 are connected by conductor 2" and 2, respectively, with the opposite terminalsof transformer secondary winding 208. Cathode 2| 2 is connected by conductors 2| and 2I8 to opposite terminals of transformer secondary winding 201. 'Wind- I .between conductors 2i! and '22l in order to remove undesirable alterhating components, or

.' ripples, from the power supply.

The triode 288 is provided with an an0de.225, a cathode 228 and a control electrode,- or grid. 228. The input circuit of the first amplification stage, which includes triode 208, may be traced from inputterminals 284 through-a condenser 230, a conductor'23l, grid 228, cathode 228,- a.

biasing r r 232 in parallel with a condenser 233, a co ductor 234, and conductor 2| 8 to input this input circuit variations in potential between terminals 284 and 285 are impressed between th rid 228 and cathode 228 oftriode 288.

The output circuit of the first amplification stage may be traced from conductor 22| which forms the positive terminal of the amplifier power supply through a conductor 235, a voltage dividing resistor 238, a conductor 231. a vditage divid- 288 to the center tap 251 242, anode 225, cathode 228, biasing resistor 232 and the condenser 233 in parallel therewith, and conductor 234 to conductor 2l8 which forms the negative terminal of the power circuit. In a well 5 known manner, the fluctuations of the potential applied between grid 228 and cathode 228, of the first amplifier stage are reflected in amplified fluctuations of the current flowing in the output circuit. c

The fluctuations of current in the output citcult of triode 200 cause a fluctuating potential;

'drop, across load resistor 242. The fluctuations of this potential drop across resistor 242 are trans- 1 mitted through coupling condenser 243 to the input circuit of the second amplifier stage, which includes the triode l. Triode 2 has an anode 248, agrid 241, and a cathode 248. The input circuit of triode 20l may be traced from the upper terminal of resistor 245 through grid 241, cathode 20 248, biasing resistor 250 in parallel with a condenae'r l, to conductor 234 and the lower terminal of resistor 245.

The output circuitof the second amplifier stage 25 serves as the positive terminal of the power supply through conductor 235, voltage dividing resistor 238, conductor 231, a conductor 252, condenser 253 anda primary winding 254 of a coupling transformer 255 in parallel therewith, anode 248,

so cathode 248, biasing resistor 250 and condenser 25| in--parallel therewith, and conductor 234 to conductor 2l8 which serves as the negative terminal of the power supply. In a well known manner, the fluctuations in the potential im-' pressed on the input circuit of triode 20l are reflected in even greater fiuctuations'in the current flowing in the output circuit.

The output circuit fluctuations flowing in f the transformer primary winding 254 produced 40 similar fluctuations in a secondary winding 258 on the transformer 255.

The secondary winding 258 is connected in the input circuit of the final push-pull amplification stage including the twin triode 202. The

5 twin triode 202 comprises a, pair of anodes 280 and 28l, a pair of control electrodes 282 and 283, each associated with one of the anodes 280 and 28L respectively. and a common cathode The input circuit for the upper half of I gojtwin triode 202 may be traced from the upper 1 terminal of secondary winding 258 through a conductor 285, control grid 282, cathode 284, a

conductor 288, a resistor 281, and a conductor" 288 toth mid-point 251 of transformer second 5 my winding 258. The input circuit for the lower half of twin triode 202 may be traced from the lower terminal of secondary winding 258 through a conductor 210, control grid 283, cathode 284,

conductor 288, resistance 281, and conductor of transformer secondary winding 258.

The output circuit of the final push-pull amplification stage is divided into two branches;

One of these branches includes the upper half of the twin triode 282, and may be traced from terminal 285. It shouldbeapparent thatthrough 10 tor 215, anode 280, cathode 284, conductor 288,

and conductor 234 to the negative terminal MS of the power supply circuit. The other branch of the push-pull output circuit may be traced from positive terminal 22l of the power suping resistor 248. a'conductor 2'4l.a loadresistor 1 P y, thr u h conductor 21'. m d-po n the lower half of transformer primary winding 213, a conductor 216, anode 26l, cathode 264, conductor 266 and conductor 234 to negative terminal 2l9 of the power supply. A condenser 230 is connected across the terminals of transformer primary winding 213 for power factor correction purposes.

Th push-pull amplifier stage operates in the conventional manner of such circuits to produce a greatly intensified fluctuation of the current in its output circuit in response to small variations in the potential supplied to its input circuit. This fluctuating output circuit flowing in the transformer primary winding 213 produces similar fluctuations in transformer secondary winding 211, which is connected to the output terminals 286 and 281 of the amplifier 221.

It has been found that amplifiers of the type described herein have a characteristic shift in phase of the fluctuation in the output current with respect to the phase of the fluctuation of the input potential as the magnitude of those fluctuations increases. A certain shift in phase takes place in each stage of the amplifier. The phase shift between the first stage input terminals and the final stage output terminals is the resultant of the individual phase shifts in each stage. When a multiple stage amplifier is used, and the phase shifts of successive stages are in the same sense. a condition may occur wherein the resultant phase shift through the entire amplifier is very high, being greater than 90 electrical degrees and in extreme cases approaching 180 degrees. In the present control system, where the amplifier is to be used in connection with a motor control circuit for a split-phase motor as described. for example, in the system of Figure 1, it is necessary that means be provided for limiting this shift in phase to less than 90 electrical degrees. If means were not provided for limiting the shift in phase, a large fluctuation in the input potential might cause the phase of the output current to shift so much that the direction of rotation of thesplit-phase motor would be reversed, thereby producing erratic operation of the control system.

We have therefore provided in the amplifier circuit of Figure 2 means for limiting the volume of the final push-pull amplification stage in order that the phase shift of the output may also be limited. This means includes the resistor 261 which is connected in both input circuits of the twin triode 202, and has established across it, a potential drop which is proportional in magnitude to the amplitude of the fluctuations applied to control grids 262 and 263. This potential drop is fed back to the input circuit of the first amplification stage. This feed-back circuit may be traced from the left-hand terminal of resisto r 261 through a conductor 280. a filtering resistor 28!, a conductor 283, a resistor 280, conductor 23!, grid 228, cathode 226, resistor 232 and condenser 233 in parallel therewith. and conductor 234 to the right-hand terminal of resistor 261. A filtering condenser 282 is connected between conductors 283 and 234.

The fiow of current in the input circuit of the push-pull amplification stage produces a potential drop across resistance 261 such that the right-hand terminal is more positive than the left-hand terminal. as indicated by the legend in the drawings. The left-hand terminal of resistor 261 is connected through the circuit described above to control grid 228, while the righthand terminal of resistor 231 is connected to the cathode 226. It should therefore be apparent that as the output of the amplifier increases. the potential drop across resistor 261 increases and an increasing negative bias is applied to grid 228. This negative bias produces a counter-active effect tending to reduce the output of the amplifier. By properly proportioning the circuit elements, such as resistor 261, this negative bias effect may be made to definitely limit the amplifier output at some predetermined value, thereby limiting the shift in phase of the output current with respect to the input potential.

This limiting of the amplifier output current does not adversely efiect the motor control systern shown in Figure 1. The only effect which might possibly be considered adverse is a limitation of the speed of that motor. Since the distance which the motor may run in a given direction is limited by the movement of the valve 20 and the slider 45, the speed of the motor, even though limited, may be designed to be ample to run the valve and slider to either of their limiting positions within an inconsequential period of time after a complete unbalance of the bridge circuit in either direction.

Figure 3 Figure 3 illustrates another form of electronic amplifier which may be used in the control system of Figure 1. The amplifier shown in Figure 3 is generally similar to that in Figure 2 and is indicated by the reference number 321. All elements in Figure 3 which are the equivalent of corresponding elements in Figure 2 bear reference numerals in the 300 series which correspond to the reference numerals of the equivalent elements in Figure 2 in the 200 series. For example, input terminal 284 in Figure 2 is the equivalent of input terminal 384 in Figure 3.

The chief feature of novelty in Figure 3 is that. the volume control arrangement including the resistor 261 and the feed-back circuit associated therewith, has been eliminated. It should also be noted that the filtering condenser 222 has been removed from the power supply circuit in Figure 3.

By way of example, a third stage of preliminary amplification, generally indicated at 39 I has been added to the circuit of Figure 3. The possible addition of further preliminary stages is indicated by the dotted lines 392, 393 and 364. The additional stage 388 is the equivalent of the other stages including the triodes 300 and 3M, and needs no further description,

Elimination of undesirable phase shifts between the input and output terminals of the amplifier 321 has been accomplished by a much simpler means than the volume control arrangement shown in Figure 2. The replacement of the reactive coupling condenser 238 in Figure 2 by the resistance coupling element 328 in Figure 3 has been found to decrease the tendency of the system to shift the phase of the output. The phase shifting tendencies of the system have been further reduced by elimination of the filtering condenser 222. As a result of the elimination of this filtering condenser, an alternating component having twice the frequency of the primary power source exists between the terminals 318 and 32l of the power supply circuit. This double frequency component is in phase with the primary power supply potential. It should be noted that this power supply including the double frequency component is through conductors 32I and 334 without any filtering whatsoever. As long as the current flowing in the output circuit of the push-pull amplification stage remains in phase or 180 degrees out of phase with the primary supply voltage, this double frequency component has substantially no effect. The double frequency component aids the output in one half of the push-pull stage while it opposes the output in the other half. These two eirects substantially cancel each other.

If the output current in the final stage shirts in phase with respect to the double frequency component, however it opposes the shift in both halves of the push-pull stage, and thereby tends to stabilize the phase oi the ogtput current. It should therefore be apparent that the use of unfiltered current from the rectifier provides a means 01' preventing undesired shifts in the phase the output current.

Figure 4 Figure 4 discloses a system for supp yin air to and controlling the temperature oi a pair of zones I00 and II. Conditioned air is supplied to the zone I00 through a duct I02, while conditioned air is supplied to the zone IOI through a duct I03. The ducts I02 and I03 are joined to form a common supply duct I 04.

Air is forced through the supp duct I04 by a. fan schematically indicated at I05. Fresh air may be drawn from the outside of the building in which zones I00 and IOI are located through a supply duct I05 to the intake of fan I05. The air in the zones I 00 and IOI may be drawn through return air ducts I01 and I03, respectively. The ducts I01 and I03 Join to form a common return air duct I00 which is also connected to the intake of the fan I05.

The relative proportions of fresh and recirculated air which are supplied to the fan intake are controlled by mixing dampers II2 operated by a motor mechanism I I3.

The motor mechanism I I3 includes a motor I 0! the split-phase type having a pair of windings I3I and I32, whose common terminal I33 is connected to ground. Operation of motor I30 is controlled in accordance with the unbalance of a bridge circuit generally indicated at I34 and including a temperature responsive resistance element I35 exposed to the temperature of the air near the point of discharge from the fan I05.

Bridge circuit I34 has input terminals I33 and I31 and output terminals I33 and I39. Output terminal I33 is the point of engagement of a slider I with a slide wire resistance I4I. Slider I40 is moved along slide wire I H by operation of motor I30 so that the position of slider I40 is varied in accordance with the position of mixing dampers II2.

Electrical energy is supplied to the bridge circuit I34 by a transformer I42 having a primary winding I43 connected to suply lines I44 and I45 and a secondary winding I43 connected to bridge input terminals I36 and I31.

The upper left-hand arm of bridge circuit I34 connects input terminal I 33 with output terminal I33, and includes a conductor I50, temperature responsive resistance element I35, a conductor I5I, and that portion of slide wire resistance I4I between its lower terminal and output terminal I33. The upper right-hand arm or bridge circuit I34 connects output terminal I33 with input terminal m and includes that portion or resistance I4I between output terminal I33 and the upper connected to the push-pull amplification stage.

terminal oi resistance I4I. a conductor I52, and a fixed resistance I53.

The lower left-hand arm of bridge circuit I34 connects input terminal I38 with output terminal I33 and includes a fixed resistance I54. The lower right arm of bridge circuit I34, which connects output terminal I39 with input terminal I31, includes a fixed resistance I and a variable resistance I53. The function of variable resistance I53 is to determine the value of temperature adjacent sensitive element I35 which causes the bridge circuit to be balanced. Output terminal I35 is connected through conductors I51 and I53 to input terminal I34 0! an amplifier I21. Output terminal I 33 is connected through a conductor i5, a stationary contact I30, a movable switch arm I3I, and a. conductor I32 to the other input terminal I35 of amplifier I21.

A transformer 440 has a primary winding 4 connected to supply lines I44 and I45,- and a pair of secondary windings 442 and 443. Secondary winding 443 is connected to amplifier power supply terminals I33 and I83.

The right-hand terminal of winding 442 is grounded, as at 441. Winding 442 supplies electrical energy to winding I3I of motor I30, Winding I3I is continuously energized through a circuit which may be traced from the left-hand end oi. secondary 442 through conductors 444 and 445, a condenser 443, winding I3I, and ground connections I33 and 441 to the right-hand terminal of secondary 442.

Winding I32 may be energized by the output current of amplifier I21. The energizing circuit for winding I32 may be traced from amplifier output terminal I31, through conductors 450 and 45I, a switch arm 452, a stationary contact 453, a conductor 454, winding I32. ground connections I33 and 441, and a conductor 455 to amplifier output terminal I35.

Switch arms ISI and 452 are operated by an actuating mechanism generally indicated at 443. to be described later. so as to engage contacts I30 and 453, respectively. at the same time. By this mechanism, bridge circuit I34 is connected to the amplifier input circuit when winding I32 of motor mechanism I I3 is connected to the amplifier output circ'uit. r

The common supply duct I04 is separated longitudinally by a partition II4. Where the duct I04 separates into the zone supply ducts I02 and I03, the partition II4 divides into corresponding partitions H5 and H5, each of the latter continuing in the zone supply ducts I02 and I03, respectively. On opposite sides of the partition I I4 in the common duct I04 are mounted a cooling coil I20 indicated by the legend C. C. in the drawings, and

a heating coil I2I indicated by the legend H. C. in

the drawings. The cooling coil I20 is supplied with cooling fluid through a valve I22, which may be controlled in any suitable manner. The heating coil I2I is similarly supplied with heating fluid through a valve I23, which may be controlled in any desirable manner. The air cooled by the cooling coil I20 flows through the ducts I 04, I02 and I03 on the left-hand side of the partitions I I4, I I5 and I I3. respectively. On the other hand. air heated by the heating coil I2I flows through the ducts I02, I03 and I04 on the right-hand side of the partitions H5, H5 and H4, respectlvely.

The relative amounts of heated and cooled air supplied to zone I03 are controlled by a mixing damper I24 which is operated by a motor mechanism I25. The relative amounts of heated and cooled air supplied to the zone IOI are controlled by a mixing damper I26 operated by a motor mechanism I28.

Motor mechanism I25 includes a motor 165 of the split-phase type, having a pair of windings I66 and I61 whose common terminal is grounded as at I68. Operation of motor I65 is controlled by a bridge circuit generally indicated at I15, having input terminals I16 and I11 and output terminals I18 and I19. Bridge circuit I15 includes a first temperature responsive resistance element I80 exposed to the air temperature in the discharge duct I02, a second temperature responsive element I8I exposed to the temperature in the return air duct I01, and a third temperature responsive element I82 locatedin the fresh air supply duct I06.

Electrical energy is supplied to bridge circuit I15 by a transformer I83 having a primary winding II8 connected to supply lines I44 and I45,

and a secondary winding II9 connected to input terminals I 16 and I11 by conductors I1! and I12, respectively.

The upper left arm of the bridge circuit I 15 connects input terminal I16 with output terminal I19 and includes temperature responsive resistance element I80, a conductor I13, a conductor I14, and temperature responsive resistance ele ment I82.

The upper right-hand arm of bridge circuit I15 connects output terminal I19 with input terminal I11 and includes a conductor I90, a variable resistance I9I, a fixed resistance I92, and a conductor I93.

The lower left arm of bridge circuit I15 connects input terminal I16 with output terminal I18 and includes a conductor I41, a fixed resistance I48, a conductor I49, and that portion of a slide wire resistance I94 between the left-hand terminal and output terminal I18. Output terminal I18 is the point of contact between a slider I95 and slide wire I94. Slider I95 is moved along slider I94 by motor I65 as the latter moves mixing dampers I24. The position of slider I95 therefore serves as a measure'of the position of dampers I24.

The lower right arm of bridge circuit I15 connects output terminal I18 with input terminal I11 and includes that portion of the resistance wire I94 to the right of output terminal I18. a conductor I96, a conductor I91. and temperature responsive resistance element I8I Output terminal I18 is connected to input terminal I84 of amplifier I21 by means of conductor I58. Output terminal I19 is connected to input terminal I85 of amplifier I21 through conductor I98, stationary switch contact I99, a switch arm I69, and conductors I10 and I62.

Transformer secondary winding 442 supplies energy continuously to winding I86 of motor I65. The energizing circuit may be traced from the left-hand end of secondary 442 through conductor 444, a condenser 456, winding I66, and ground connections I68 and 441 to the right hand end of winding 442.

Winding I61 may be energized by the output current of amplifier I21. The energizing circuit for winding I61 may be traced from amplifier output terminal I81 through conductor 450, a switch am 451, a contact 458, a conductor 460, winding I61, ground connections I68 and 441, and conductor 455 to amplifier output terminal I86.

Switch arms I69 and 451 are operated by actuating mechanism 449 so as to engage contacts I99 and 458, respectively, at the same time. When the switch arms I68 and 451 are engaging contacts I99 and 468, respectively, bridge circuit I15 is connected to the amplifier, input circuit and winding I61 of motor I65 is connected to the amplifier output circuit. I

Motor mechanism I28 includes a motor 400 o the split-phase type having a pair of windings 4M and 402 with their common terminal 403 grounded. Operation of motor 400 is controlled by a bridge circuit 404, having input terminals 405 and 406 and output terminals 401 and 408. Bridge circuit 404 includes a first temperature responsive resistance element 0 exposed to the temperature in the discharge duct I03, a second temperature responsive resistance element 4| I exposed to the temperature in return air duct I08, and a third temperature responsive resistance element 4I2 exposed to the temperature in fresh air supply duct I06.

Electrical energy is supplied to the bridge circuit 404 by a transformer 4I3, having a primary winding 4I4 connected to supply lines I44 and I45, and a secondary winding 5 connected to input terminals 405 and 406 by conductors M6 and H1, respectively.

The upper left-hand arm of bridge circuit 404 connects input terminal 405 with output terminal 408 and includes temperature responsive resistance element 4I0, a conductor 420, a conductor 42I, and temperature responsive resistance element H2.

The upper right-hand arm of bridge circuit 404 connects output terminal 408 with input terminal 406 and includes a conductor 422, a variable resistor 423, a fixed resistor 424, and a conductor 425.

The lower left-hand arm of bridge circuit 404 connects input terminal 405 without terminal 401 and includes a conductor 426, a fixed resistance 421, a conductor 428, and that portion of a slide wire resistance 430 to the left of the point of engagement of a slider 43I therewith.

This point of engagement constitutes the output terminal 401 of bridge circuit 404. The slider 43I is moved along slide wire resistance 430 by operation of motor 400, so that the position of slider 43I serves as an indication of the position of damper I26.

The lower right-hand arm of bridge circuit 404 connects output terminal 401 with input terminal 406 and includes that part of slide wire resistance 430 to the right of slider 43I, conductors 432 and 433, and temperature responsive resistance element M I.

Output terminal 401 of bridge circuit 404 is connected to input terminal I84 of amplifier I21 through a conductor 434 and conductor I58. Output terminal 408 of bridge 404 is connected to amplifier input terminal I85 through a conductor 435, a switch contact 436, a switch arm I69, and conductors I10 and I 62.

Transformer secondary winding 442 supplies energy continuously to winding 40I of motor 400. The energizing circuit may be traced from the left-hand end of secondary 442 through conductors 444 and 445, a conductor 46I, a condenser 462, winding 40I, and ground connections 403 and 441 to the right-hand terminal of secondary winding 442.

Winding 402 may be energized by the output current of the amplifier I21. The energizing circuit for winding 402 may be traced from amplifier output terminal I81 through conductor 450, switch arm 451, contact 463, a conductor 464, winding 402, ground connections 403 and 441 and conductor 466 to amplifier output terminal I66.

Switch arms I" and 461 are operated by actuating mechanism 449 so as to engage contacts 436 and 463, respectively, at the same time. When the switch arms I68 and 451 are engaging contacts 438 and 463, respectively, bridge circuit 404 is connected to the amplifier input circuit and winding 402 of motor mechanism I28 is connected to the amplifier output circuit.

Switch actuating mechanism 449 includes a continuously operating motor 465 which may be connected to power supply lines I44 and I45. Motor 466 drives a pair of similar cams 466 and 461. Cam 466 cooperates with switch arms 452 and 461, thereby controlling the connection of the various motors to the amplifier output circuit. Cam 461 cooperates with switch arms I6! and I69, thereby controlling the connection of the various bridge circuits to the amplifier input.

Each switch arm is biased for engagement with its associated cam.

Cams 466 and 461 have corresponding high dwell portions 468 and 469, low dwell portions 410 and 41!, and intermediate dwell portions 412 and 413. The cams rotate simultaneously. When the high dwell portions 468 and 469 engage switch arms 451 and I69 respectively, those, switch arms are moved into engagement with contacts 458 and I99. When the low dwell portions 410 and 41! engage switch arms 451 and I68, the latter engage contacts 463 and 436 respectively. When the intermediate dwell portions 412 and 413 engage switch arms 451 and I69, the switch arms are held in an intermediate position, engaging neither of their respective contacts. In a similar manner, switch arms 452 and I6! are simultaneously operated to engage contacts 453 and I60. Contacts 416 and 411 are provided adjacent the switch arms 452 and I6! respectively, so that another condition responsive bridge circuit and an associated condition controlling motor could be added to the system if desired.

A variable resistance 48!] is connected in parallel with temperature responsive element 4 I 2, and a similar variable resistance 48! is connected in parallel with temperature responsive element I82. Resistances 460 and 48! are of the slide wire type. Their effective values are varied by a pair of sliders 482 and 483 which are mechanically interconnected so as to be operated by a single temperature responsive device 484, shown in the drawings as a bellows, is exposed to the temperature in the fresh air intake I06 wherein he temperature responsive resistance elements M2 and I82 are mounted.

The function of temperature responsive resistance elements M2 and I82 is to compensate their respective bridge circuits for the temperature of the fresh air admitted to the system. The purpose of this compensation is to provide, during extremely hot weather, an increase in the indoor temperature in proportion to the increase in outdoor temperature. Such compensation has two distinct advantages. In the first place, such compensation prevents the occurrence of an excessive difference between indoor and outdoor temperatures which might cause undue shock to persons entering or leaving the building. In the second place, such compensation provides for more economical operation of the cooling system. It has been found that when the outdoor temperature is 100, for example, people are sufliciently comfortable when the indoor temperature is maintained at On the other hand, when the outdoor temperature is 85, the same people will require that the indoor temperature bemaintained at '15 in order to secure the same comfort. Therefore by operating the system at 85 when the outdoor temperature is at 10!), rather than maintaining the inside temperature constantly at 75, a considerable saving in the amount of energy necessary to cool the interior of the building is secured.

This compensation for outdoor temperature should not be present when the outdoor temperature is below a certain value, for example 75. Furthermore, the amount of compensation should vary with the outdoor temperature. That is, a change in outdooricmperature from 75 to 80 should not cause as great a change in the control point oi the inside temperature as a change in outdoor temperature irom to We have taken care of these changes in compensation by introducing the variable resistances 480 and 48!. The sliders 482 and 483 are operated by the bellows 484 so that they are at the lower end of their associated resistances whenever the outside temperature is lower than 15. The resistances M2 and I82 will therefore be shunted from their respective bridge circuits at such a time. As the outdoor temperature increases above 75 the sliders 482 and 483 move upward along their associated resistances 480 and 48!. Thus as the temperature increases an increasing amount of resistance is inserted in parallel with the elements 4 I 2 and I82. A given change in resistance of these latter elements therefore has an increasing effect on the unbalance of the system. When the sliders 482 and 483 reach the upper end of their slide wires, there is no further change in the amount of compensation of the inside temperature for each degree change in the outside temperature. Alter such a condition is reached, the change of outdoor temperature aifects only the elements M2 and I82, and the compensation of the bridge circuit depends only on the temperature coefficient of resistance of those elements.

It should be apparent that the mechanism just described provides a. non-linear compensation of the bridge circuit, varying from zero at values below 70, through a range from, for example, 70 to 100", where the rate of compensation increases with temperature to a second range, for example, above 100 where the rate of compensation is constant. 7

Operation of Figure 4 In describing the operation of the air conditioning system shown in Figure 4, the bridge circuit I34 and its control of the mixing dampers I I2 will first be described.

Let it be assumed that the motor 465 is stopped in such a position that switch arms 452 and I6! are in engagement with contacts 453 and I60, respectively. Motor winding I32 is then connected to the output circuit of amplifier I21 and bridge circuit I34 is connected to the input circuit of amplifier I21. Variations in the balance of bridge circuit I34 caused by changes in the temperature adjacent sensitive element I35 then produce unbalance potentials in the bridge circuit which are applied to the input terminals of the amplifier. The output of the amplifier is fed to winding I32 so as to cause rotation of the motor I30 in the proper direction to move slider I40 in such a direction as the rebalance bridge circuit I34. As the slider I4!) is moved, the'mixing dampers II2 are simultaneously moved so as to change the proportions of fresh air and recirculated air supplied to the system The change in position of dampers III will be'in such a diree. tion as to correct the air temperature in the duct I84, returning it to the value which the system was set. to maintain by adjustment of the variable resistance III.

For example, under summer conditions the outside air supplied through the duct II! will normally be at a higher temperature than the recirculated air supplied through the duct I88. It is desired to maintain the air in the duct I at a predetermined temperature so that too great a load will not be placed on the cooling coil I28. By adjusting resistance I88 the bridge circuit I84 may be made to balance at any predetermined temperature adjacent the sensitive resistance element I88. If the temperature in the duct I should increase above this value the increase in resistance of element It! causes a current to flow in the amplifier input circuit, thereby producing a current in the amplifier output which energizes winding I82. The phase relation of this current will be such as to operate motor I88 in the proper direction to drive slider I" downwardly along resistance Ill, thereby reducing the resistance in the upper left arm of bridge circuit I" and rebalancing the bridge circuit. At the same time the dampers I I2 will be operated in such a direction as to decrease the supply of relatively hot outside air and increase the suppl of relatively cool recirculated air, therebytending to return the temperature in duct I84 to its predetermined value.

In a similar manner, a fall in temperature in the duct I causes operation of the dampers II2 to increase the supply of relatively warm outside air and decrease the supp y of relatively cool recirculated air.

Since the temperature of the outside air in, the winter will be normally colder than that of the inside air, this control may be reversed in the winter so as to increase the proportion of recirculated air when the temperature in duct I04 decreases, and to increase the proportion of fresh air when the temperature in duct I increases. This reversal of function may be produced in any desired manner. By way of example, one way of accomplishing this result would be to reverse the ungrounded terminal connections of motor winding I32. It will be readily understood by those skilled in the art that suitable means may be provided to establish a suitable minimum position of the fresh air dampers H2.

The operation of bridge circuit I15 and the motor mechanism I25 controlled thereby will next be described. For this purpose let it be assumed that motor 465 is stopped in such a position that switch arms 45'! and I88 are engaging contacts 458 and I98 respectively, thereby connecting motor I65 to the output circuit of the amplifier and connecting bridge I15 to the amplifier input circuit. Bridge I15 includes three temperature responsive elements I80, Ill and I82. Temperature responsive element I80 is exposed to the temperature of the air being discharged into the space I88 through the duct I82. Temperature responsive element I8I is responsive to the temperature being returned from the space I to the air conditioning system through the duct I01. Temperature responsive element I82 is'exposed tothefmhairbeing supplied to the system through the duct I86.

Adecreaseinresistanceotanyoneofthese asraoss temperature responsive elements indicates a present or impending decrease in the temperature of space Ill. Since both elements I" and I88 are connected in the upper left arm of the bridge, while the resistance I82 is connected in the opposite lower right arm of the bridge, an increase in resistance of any one produces an unbalance of the bridge circuit in the same direction. Such an unbalance produces a current fiow in the amplifier input circuit which is reflected in an increased flow in the amplifier out.- put circuit in such a direction that motor I88 operates dampers l2 so as to decrease the supply of cooled air and/or increase the supply of heated air, as the'case may be. At the same time slider I8 is driven to the right along slide wire Ill so as to rebalance the bridge circuit I18.

In a similar manner a decrease in temperature of any one of the resistance elements I", Ill or I82 causes operation of the motor I88 in such a direction as to operate mixing dampers I24 to increase the supply of heated air or decrease the supply of cooled air. Simultaneously slider I8! is driven to the left along resistance ill to rebalance bridge circuit I18.

Bridge circuit 484 is provided with temperature responsive elements ill, II and I2 which correspond to the resistance elements I80, iii and I82 in bridge circuit III. Bridge circuit till acting through motor mechanism I28 controls the air supply to zone III in the same manner that bridge circuit I15 controls the supply of air to zone IIIII. Further description of the operation of bridge circuit 484 is believed to be unnece It has been found that in a control system of the type described, if the bridge is continuously connected to the condition controlling motor, the motor will operate only during about five minutes out of every hour. Since amplifier circuits of the type necessary in our motor control system are quite expensive, we have provided a switching mechanism which takes advantage of this operating characteristic so that a single amplifier may be used to connect successively various bridge circuits to their associated motors. This arrangement may be used in any control system as long as the number of control circuits is kept small in proportion to the average proportion oi time each control motor would run if continuously connected to its amplifier. For instance, in the present system it has been stated that each control motor would run approximately five minutes out of every hour if continuously connected to an amplifier. Therefore, since each control motor runs approximately one-twelfth of the time the number of control circuits which may be usedon one amplifier should be made smaller than twelve. It has been found that four is a satisfactory number and provides a suiiicient factor of safety to take care of any unusual conditions which may arise.

While we have shown and described preferred embodiments of the various features of our invention, other modifications will readily occur to those skilled in the art, and we therefore wish to be limited only by the scope of the appended claims.

We claim as our invention:

1. A system for controlling the temperature of the air in a space, comprisin in combination, temperature changing means, return duct means forconveyingairfromsaidspacetosaid temperaturechangingmeans, freshairduct means for conveying outside air to said temperature changing means, discharge duct means for conveying air from said temperature changing means to said space, damper means in 'said discharge duct, motor means for operating said damper means, a resistance bridgecircuit having a first element with an appreciable temperature coefficient of resistance and exposed to the air temperature in said return duct, a second element with an appreciable temperature coefficient of resistance and exposed to the air temperature in said fresh air duct, and a third element with an appreciable temperature coeiiicient of resistance and exposed to the air temperature in said discharge duct, means for applying an alternating voltage to said bridge circuit, and means responsive to the phase relationship between the output voltage of said bridge and said first named alternating voltage for controlling said motor.

2. In an air conditioning system for a space, in combination, air conditioning means, means for mixing and circulating air through said conditioning means, return air duct means for conveying air from said space to said circulating means, fresh air duct means for conveying outside air to said circulating means, damper means for proportloning the relative amounts of air passing through said fresh air and return air ducts, motor means for operating said damper means, a resistance bridge circuit having an element with an appreciable temperature coefiicient of resistance exposed to the temperature of the air discharged from said mixing and circulating means, means for applying an alternating voltage to said bridge circuit, and means responsive to the phase relationship between the output voltage of said bridge and said first named alternating voltage for controlling said motor.

3. In an air cooling system, in combination, duct means for supplying outside air to said system, air cooling means, control means for said air cooling means, means for operating said control means including a bridge circuit, a first element in said circuit having an appreciable temperature coefficient of resistance and exposed to a temperature indicative of the need for operation of said control means, a second element in said circuit having an appreciable temperature coeilicient of resistance and exposed to the air temperature in said duct means, said element compensating said circuit for variations in said last-mentioned air temperature, a rheostat connected in parallel with said second element to limit its compensating efiect, and means responsive to said last-mentioned air temperature for operating said rheostat.

4. In an air cooling system, in combination, duct means for supplying outside air to said system, air cooling means, electrical motor means for operating said air cooling means selectively in opposite senses in accordance with the phase of an electrical potential supplied thereto, an electronic amplifier including an input circuit and an output circuit and having a predetermined normal phase relationship between potentials in said output circuit and said input circuit, means connected to said amplifier for substantially preventing a disturbance of said normal phase relationship, connections between said output circuit and said motor means, a source of alternating electrical energy, a bridge circuit having input terminals and output terminals, a first element in said bridge circuit having an appreciable temperature coeilicient of resistance and exposed to a temperature indicative of the need for operation of said control means, a second element in said bridge circuit having an appreciable temperature coefiicient 01' resistance and exposed to the temperature in said duct means, said elerr compensating said circuit for variations in last-mentioned air temperature, a rheostat 5 nected in parallel with said second elemen limit its compensating effect, means responsiv said last-mentioned air temperature for ope ing said rheostat, and a connection between output terminals and said amplifier input cin 5. A temperature control system comprisin combination, a bridge circuit including an ment of fixed length whose specific resist: changes with temperature and which is exp to a temperature condition affecting 'said sysi an adjustable resistance so connected in par; with said element across the extremities the that the adjustment of said resistance varies resistance in parallel with the terminals of element, a thermostatic device responsive to same temperature to which said element is posed for adjusting said resistance, tempera changing means, and means responsive to balance of said bridge circuit for controlling temperature changing means.

6. In a temperature responsive electrical work, a first element whose specific resist: changes with temperature and which is exp to a first temperature, said element being 1 nected in said network so as to produce unbali potentials in said network in accordance i changes in said first temperature, a second ment connected in said network and whose cific resistance changes with temperature which is exposed to a second temperature so i compensate said unbalance potentials in ace ance with changes in said second temperatui rheostat comprising a resistance element a! slider in movable engagement with said resisti element, one terminal of said second element 40 lng connected to one terminal of the resist:

element of the rheostat and the other term being connected to said slider, and thermosi means exposed to said second temperature moving said slider so as to vary said com sation.

7. An air conditioning system for an enclo: comprising in combination, air conditioi means, discharge duct means for conveying from said air conditioning means to said en sure, supply duct means for conveying air to air conditioning means, return duct means conveying air from said enclosure to said su duct means, fresh air duct means for conve: air from the outside of said enclosure to supply duct means, first damper means for trolling the fiow of air through said disch: duct means, first and second bridge circ' means including a source of potential for e: gizing said first and second bridge circuits, electrical motor means for operating said damper means selectively in opposite sense accordance with the phase relation betweer electrical potential supplied therethrough said source of potential, said first bridge cii including three elements having appreciable t perature coefiicients of resistance, one of said ments being located in said discharge duct me a second in said return duct means, and

third in said fresh air duct means, second dan means for controlling the proportions of re air and fresh air entering said supply duct me second electrical motor means for operating second damper means selectively in opposite rections in accordance with the phase relation tween an electrical potential supplied thereto including an element having an appreciable temperature coefllcient of resistance and located in said supply duct means, and means including electronic amplifier means for connecting said first and second bridge circuits respectively with said first and second motor means to supply to said motor means electrical potentials whose phase relationship with respect to said source is dependent upon the direction of unbalance or said bridge circuits.

8. Control apparatus, comprising in combination, a load device to be controlled, an electrical network, means responsive to an electrical potential in said network for controlling said load device, and means for producing in said network an electrical potential variable as a non-linear function or a condition indicative of the need for operation of said load device, said last-named said source 015 potential, said second bridge circuit means comprising first impedance means connected in said network and variable as a substantially linear function of said condition and second impedance means, each 01 said first and second impedance means including an impedance element, one terminal oi the impedance element of said second impedance means being connected to one terminal 01' the impedance element of said first impedance means, said second impedance means having a movable tap connected to the other terminal 01' the impedance element of said first impedance means and movable with respect to the impedance element of said second impedance means as a substantially linear function of said condition simultaneously with a change in the impedance value of said first impedance means.

WILLIS H. GILLE.

JOHN V. SIGFORD. 

